Cardiac signals are used to measure the health of a patient's heart muscle and are typically obtained from patients by physicians using electrocardiogram (ECG) tracings. FIG. 1 illustrates a portion of an ECG tracing of two complete cycles of the cardiac signal. Each cardiac signal comprises five major signal portions that are identified respectively as the P, Q, R, S and T waves. The P wave represents electrical depolarization of the atrium. The Q wave represents the initial stages of ventricle depolarization and the R wave represents the peak of the depolarization of a ventricular myocardium. The S wave represents the final stages of ventricular depolarization and the T wave represents ventricular repolarization. The QRS complex of the ventricle generally masks atrial repolarization. Under conditions of normal cardiac activity, the R wave represents the conducted sinus beat of the heart. The interval between each apex of consecutive R waves represents the heart period in units of time, with the heart rate being the reciprocal of the RR interval expressed as beats per minute.
FIG. 1 further illustrates a normal cardiac signal labeled with two Q–T intervals, Q–T1 and Q–T2. Each Q–T interval is usually measured from the beginning of the Q wave to the end of the T wave. Studies indicate that the duration of the Q–T interval may be an indicator of cardiac electrical abnormalities. A prolonged Q–T interval, under certain conditions, can indicate a risk of sudden cardiac arrest. Electrical heterogenerty is a substrate for ventricular arrhythmia. A prolonged Q–T interval can also indicate that a ventricular arrhythmia (i.e., an irregular heartbeat or rhythm) in the patient's heart may be imminent. Left untreated, sustained ventricular arrhythmia can detrimentally impact a patient's health.
Implanted medical devices exist today that are capable of detecting and treating arrhythmia of a patient. In one example, the implanted medical device includes a defibrillator that applies an electrical therapy to a patient's heart upon detecting an atrial fibrillation. Cardioverters or defibrillators discharge relatively high energy electrical shocks across cardiac tissue to arrest a life threatening atrial or ventricular fibrillation that is detected by the implanted medical device. Defibrillation shocks, while highly effective at arresting the fibrillation, may cause considerable patient discomfort.
Accordingly, there is a need for a method for terminating an arrhythmia that will not cause patient discomfort, the effectiveness of which can be easily verified by analyzing the patient's cardiac signal. An approach that addresses the aforementioned problems, as well as other related problems, is therefore desirable.